Microsoft Corporation (NASDAQ:MSFT) has set a target of one billion devices to be running Windows 10. However, hitting this target isn’t the only thing the company should worry about.

The one billion target was set before the launch of Windows 10 last April. According to Terry Myerson, the company’s vice president for operating systems, Microsoft expected one billion devices to be running Windows 10 within two to three years after the release.

This implies that close to 330million new devices should be running Windows 10 each year until 2018. With this week’s announcement from Microsoft that it has reached 300million new devices in nine months, it is clearly evident that the company is ahead of its target.

Where is this growth coming from?

Windows 7 and Windows 8 are one area of increase as many users can upgrade to Windows 10 which is currently a free download. According to NetMarketShare’s data, there is close to 1.6 billion PCs in use, and Windows 10 accounts for 15% of the systems accessing the web. XP accounts for 11% while Windows 8 grabs 10%. The largest share goes to Windows 7, which holds 48%.

The offer to upgrade to Windows 10 is not available to Windows Vista, Windows XP, or Enterprise editions of Windows. However, since Windows 8 and Windows 7 have many users, this means large numbers of upgrades.

Since the free upgrade offer is ending on July 29th, after which the users will have to purchase a full version of Windows 10 or buy a new device to get it, it is expected that most people will upgrade.

Also, Majesco Entertainment Co.(NASDAQ:COOL) with 20 million sales to date and the HoloLens which is still being developed, tablets and phones are all included in the total.

Microsoft’s Lumia have dropped, and there is no much lift expected from this. Therefore, if Microsoft was counting 100 million Lumias, that could be a headache to Microsoft.

Another chunk of billion is expected from new PCs which is estimated 250 million, and it is anticipated that these PCs will be using Windows 10.

It seems that Microsoft has a decent chance of hitting its one billion target unless this rate of adoption slows down.